Aircraft fuel is often considered as a high octane fuel that may be used in a high performance car engine. When talking to the engineers at one of the major refineries I was rather shocked to find that they use a totally different octane rating system. Aircraft engines are rarely ran over 2500 RPM so the piston pressure is changed far less than that of the auto engine that has a wide RPM change. The higher the performance usually means even a higher RPM range. They also rate the octane according to the rich or lean mixture device on the dash of the airplane. The aircraft fuels octane ratings were considerably lower compared to the automotive fuel rating system and should never be used for automotive applications. In fact, one of the new big things for aircrafts is to use automotive unleaded gas.
In conclusion, I have personally found that people who use heads with the proper parts and stay with the correct octane rating for their engine's compression ratio, the heads live a normal life and do not experience any cooling, cracking or overheating problems. Consider this, if it required 91 octane for the stock Beetle with 7.2 or 7.3 CR, it is obvious that 85 or 87 octane will require a CR in the area of 6.6 to 6.9. By the way, my 1982 VW Beetle, as do the 1992's, came stock, new from the VW factory with 6.6 and ran cool and powerful.
Other information I make available related to this and may also be of interest is GB 801-COOLING and GB 801-CR (addresses and phone numbers of people that carry fuels that work). Both are about what is needed for the engine to live its longest.